Lighting devices with prescribed colour emission

ABSTRACT

Optical conversion layers based on semiconductor nanoparticles for use in lighting devices, and lighting devices including same. In various embodiments, spherical core/shell seeded nanoparticles (SNPs) or nanorod seeded nanoparticles (RSNPs) are used to form conversion layers with superior combinations of high optical density (OD), low re-absorbance and small FRET. In some embodiments, the SNPs or RSNPs form conversion layers without a host matrix. In some embodiments, the SNPs or RSNPs are embedded in a host matrix such as polymers or silicone. The conversion layers can be made extremely thin, while exhibiting the superior combinations of optical properties. Lighting devices including SNP or RSNP-based conversion layers exhibit energetically efficient superior prescribed color emission.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplications No. 61/299,012 filed Jan. 28, 2010 and titled “Light sourcewith prescribed colour emission” and 61/299,018 filed Jan. 28, 2010 andtitled “Phosphor-nanoparticle combination”, both of which areincorporated herein by reference in its entirety.

FIELD AND BACKGROUND

Embodiments of the invention relate in general to optical devices whichcomprise semiconductor nanoparticles and in particular to lightingdevices which include conversion layers having semiconductor quantumconfined nanoparticles.

Light emitting diodes (LED) offer significant advantages overincandescent and fluorescent lamps with respect to their high energyefficiency and long lifetimes. LEDs are applicable in diverseapplications including displays, automobile and signage lighting anddomestic and street lighting.

A LED can emit monochromatic light in different regions of the spectrum,depending on the inorganic semiconductor compound used to fabricate it.However, “white” light, which is required for a very large portion ofthe lighting industry, cannot be generated using a conventional LED.Current solutions of producing white light include the use of three ormore LEDs with various colours (e.g. Red, Green and Blue or “RGB”), orthe use of a colour conversion layer of phosphor material (e.g.Cerium:YAG) to generate a broad white spectral emission from theultraviolet (UV) or blue emission of a LED. However, such white light isalmost always non-ideal and has in many cases undesired or unpleasantcharacteristics which may require improvement or correction.

Colloidal based semiconductor quantum dots (QD) offer the possibility ofobtaining a colour gamut similar to and even better than the oneobtained with the multi-LED solution, using the narrow-band emission ofa QD tunable by size. Conversion layers incorporating ODs are known, seee.g. U.S. Pat. Nos. 7,264,527 and 7,645,397 and US patent applications2008/0173886 and 2009/0162011. However, conversion layers based on QDshave challenges. These include for example losses due to re-absorptioneffects, whereby the QD emission is reabsorbed by other QDs in thelayer. Generally this will occur for a red QD absorbing the emissionemanating from QDs which emit more to the blue. This undesired processleads to reduced energy efficiency of a regular QD conversion layer andalso to changes in the colour composition. The inherent sizedistribution of QD samples already provides different colours around acentral colour. Therefore, re-absorption will take place inherentlywithin such a layer. In devices where phosphor is used as part of alight conversion scheme to produce green light, the QD layers willabsorb partially the light from the phosphor as well, leading to bothre-absorption losses and colour changes.

In some cases, a close-packed conversion layer is desired. Close-packedQD conversion layers suffer from the phenomenon known as FluorescenceResonant Energy Transfer (FRET), see e.g. Joseph R. Lakowicz,“Principles of Fluorescence Spectroscopy”, 2^(nd) edition, KluwerAcademic/Plenum Publishers, New York, 1999, pp. 367-443. FRET occursbetween a donor QD which emits at a shorter (e.g. bluer) wavelengthrelative to an acceptor QD positioned in close proximity and which emitsat longer wavelength. There is a dipole-dipole interaction between thedonor emission transition dipole moment and the acceptor absorptiontransition dipole moment. The efficiency of the FRET process depends onthe spectral overlap of the absorption of the donor with the emission ofthe acceptor. The FRET distance between quantum dots is typically 10 nmor smaller. The efficiency of the FRET process is very sensitive todistance. FRET leads to colour change (red shift) and losses in theefficiency of light conversion.

Core/shell nanoparticles (NPs) are known. These are discretenanoparticles characterized by a heterostructure in which a “core” ofone type of material is covered by a “shell” of another material. Insome cases, the shell is grown over the core which serves as a “seed”,the core/shell NP known then as a “seeded” NP or SNP. The term “seed” or“core” refers to the innermost semiconductor material contained in theheterostructure. FIG. 1 show schematic illustrations of known core/shellparticles. FIG. 1A illustrates a QD in which a substantially sphericalshell coats a symmetrically located and similarly spherical core. FIG.1B illustrates a rod shaped (“nanorod”) SNP (RSNP) which has a corelocated asymmetrically within an elongated shell. The term nanorodrefers to a nanocrystal having a rod-like shape, i.e. a nanocrystalformed by extended growth along a first (“length”) axis of the crystalwith very small dimensions maintained along the other two axes. Ananorod has a very small (typically less than 10 nm) diameter and alength which may range from about 6 nm to about 500 nm.

Typically the core has a nearly spherical shape. However, cores ofvarious shapes such as pseudo-pyramid, cube-octahedron and others can beused. Typical core diameters range from about 1 nm to about 20 nm. FIG.1C illustrates a QD in which a substantially spherical shell coats asymmetrically located and similarly spherical core. The overall particlediameter is d₂, much larger than the core diameter d₁. The magnitude ofd₂ compared with d₁ affects the optical absorbance of the core/shell NP.

As known, a SNP may include additional external shells which can providebetter optical and chemical properties such as higher quantum yield (QY)and better durability. The combination may be tuned to provide emittingcolours as required for the application. The length of the first shellcan range in general between 10 nm and 200 nm and in particular between15 nm and 160 nm. The thicknesses of the first shell in the other twodimensions (radial axis of the rod shape) may range between 1 nm and 10nm. The thickness of additional shells may range in general between 0.3nm to 20 nm and in particular between 0.5 nm to 10 nm.

In view of the numerous deficiencies of QD conversion layers mentionedabove, there is a need for and it would be advantageous to haveconversion layers which do not suffer from such deficiencies. Inparticular, there is a need for and it would be advantageous to havenanoparticle-based thin conversion layers with negligible re-absorption(of both same and different colour), negligible clustering andhigh-loading effects and negligible FRET.

DEFINITIONS

The term “core material” to the semiconductor material from which thecore is made. The material may be II-VI, III-V, IV-VI, or I-III-VI₂semiconductors or combinations thereof. For example, the seed/corematerial may be selected from CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO,GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP,AlSb, Cu₂S, Cu₂Se, CuInS₂, CuInSe₂, Cu₂(ZnSn)S₄, Cu₂(InGa)S₄, alloysthereof, and mixtures thereof.

The term “shell material” refers to the semiconductor material fromwhich each of the non-spherical elongated shells is made. The materialmay be a II-VI, III-V IV-VI, or I-III-VI₂ semiconductor or combinationsthereof. For example, the shell material may be selected from CdS, CdSe,CdTe, ZnS, ZnSe, ZnTe, ZnO, GaAs, GaP, GaAs, GaSb, GaN, HgS, HgSe, HgTe,InAs, InP, InSb, AlAs, AlP, AlSb, Cu₂S, Cu₂Se, CuInS₂, CuInSe₂,Cu₂(ZnSn)S₄, Cu₂(InGa)S₄, alloys thereof, and mixtures thereof.

The term “host matrix” refers to a material which incorporates the SNPsor other suitable nanoparticles. The host matrix may be a polymer(formed from liquid or semisolid precursor material such as monomer), anepoxy, silicone, glass or a hybrid of silicone and epoxy. Specific (butnot limiting) examples of polymers include fluorinated polymers,polymers of ployacrylamide, polymers of polyacrylic acids, polymers ofpolyacrylonitrile, polymers of polyaniline, polymers of polybenzophenon,polymers of poly(methyl mathacrylate), silicone polymers, aluminiumpolymers, polymers of polybisphenol, polymers of polybutadiene, polymersof polydimethylsiloxane, polymers of polyethylene, polymers ofpolyisobutylene, polymers of polypropylene, polymers of polystyrene andpolyvinyl polymers, polyvinyl-butyral polymers or perfluorocyclobutylpolymers.

The term “ligand” refers to an outer surface coating of thenanoparticles. The coating passivates the SNP to prevent agglomerationor aggregation to overcome the van der Waals binding force between thenanoparticles. Ligands in common use: phosphines and phosphine oxidessuch as trioctylphosphine oxide (TOPO), trioctylphosphine (TOP),tributylphosphine (TBP), phosphonic acids such as dodecylphosphonic acid(DDPA), tridecylphosphonic acid (TDPA), octadecylphosphonic acid (ODPA)or hexylphosphonic acid (HPA), amines such as dodecyl amine (DDA),tetradecyl amine (TDA), hexadecyl amine (HDA) or octadecyl amine (ODA),thiols such as hexadecane thiol or hexane thiol, mercapto carboxylicacids such as mercapto propionic acid or mercapto undecanoic acid andother acids such as myristic or palmitic acid.

SUMMARY

Embodiments of the invention disclose optical conversion layers (orsimply “layers”) incorporating at least one SNP species and/or othernanoparticles that have the needed characteristics rendering this layerwith its unique optical properties. One such layer according to anembodiment of the invention is referred to henceforth as a “SNPconversion layer” or simply “SNP layer”. References will be madehenceforth also to “SNP sub-layers” representing part of a SNP layer,and to SNP multilayers, representing a structure with a plurality of SNPlayers. Similar terms will be used for RSNP based layers with “SNP”replaced by “RSNP”. To clarify, henceforth in this description, “layer”is equivalent to “conversion layer”. Embodiments of the inventionfurther disclose the application of SNP conversion layers fortransforming light, particularly for conversion of LED monochromaticemission of short wavelength (e.g. blue or UV) to longer wavelengths inthe VIS/NIR range to produce light of different colours. In particular,SNP layers of the invention may be used with one or more LEDs to providea white light device with high energy efficiency and good opticalproperties such as high CRI (Colour Rendering Index) and desired CCT(correlated colour temperature). In other lighting applications, a SNPlayer can provide a necessary and beneficial spectral output such aslarge gamut coverage, or specific colour bands.

In an embodiment, a SNP layer may include one type (species) of SNPemitting essentially at a single colour. In another embodiment, a SNPlayer may include a mixture of several types of SNPs emitting atdifferent colours. In some embodiments, a SNP layer may include SNPsincorporated in a host matrix, with or without ligands. A SNP layeredstructure may comprise several sub-layers, each of which may include amixture of SNPs, or may include a different type of SNP.

In some embodiments, the SNP layer thickness may be equal to or thinnerthan 200 μm. In some embodiments, the SNP layer thickness may be equalto or thinner than 50 μm. In some embodiments, the SNP layer thicknessmay be equal to or thinner than 2 μm. In other embodiments, the SNPlayer may have a thickness ranging between ca. 50 and 1000 nm.

In some embodiments, a SNP layer may include SNPs having a high-loadingratio within a polymer matrix, epoxy or resin. In some embodiments, thehigh-loading ratio may be up to 40%. In some embodiments, thehigh-loading ratio may be up to 80%. In some embodiments, thehigh-loading ratio may be up to 100%.

SNP conversion layers according to embodiments disclosed herein providefunctionalities and advantageous properties which are unknown in QDconversion layers and which were not previously discovered. Theseinclude:

1) Negligible re-absorption (both same colour and different colour). There-absorption in a SNP conversion layer is reduced significantly (incomparison to that in a QD conversion layer) because of low redabsorbance. In general, re-absorption leads to loss of energy. Forexample, assume a typical QY of 0.8. In a single re-absorption event,the OY is reduced to 0.8×0.8=0.64. In two such events, the QY is reducedfurther to 0.8³=0.51. This loss is avoided in a SNP conversion layer.Hence, the efficiency is improved. Re-absorption also leads to a redshift, which is also avoided in such a SNP conversion layer. The aspectof negligible re-absorption is present not only for one colour on itself(e.g. red to more red), but also in green emitting SNPs or phosphors.Namely, with a QD conversion layer, red QDs will reabsorb greenemission, leading to reduced efficiency and colour shift. With a SNPconversion layer, this re-absorption is minimized. Both same colour anddifferent colour re-absorption avoidance functions are unique featuresof SNP layers, whether densely-packed or not.

2) Very efficient “funneling” of energy from blue excitation to redemission. An SNP conversion layer acts essentially as an “opticalantenna” in the spectral sense. It performs this task much moreefficiently that a regular QD conversion layer, since it has very highabsorbance in the blue and strong red photoluminescence (PL) accompaniedby minimal red re-absorption, see point (1) above.

3): FRET avoidance or minimalization. In a regular QD conversion layer,The typical length scale of FRET is ˜10 nm, with 1/R⁶ dependence, whereR is the distance between two QD particles. For example, if the initialemission QY is 0.8, the QY is reduced to 0.64 after a single FRETprocess. FRET also leads to red shift, which is avoided in a SNP layer.A densely-packed SNP conversion layer (with small distances between theSNPs, e.g. 0-50 nm) owing to their unique characteristics will avoid thelosses and deficiencies related to FRET commonly encountered indensely-packed QD layers. Exemplarily, “densely-packed” as applied toSNPs in conversion layers of the invention may include ˜85% SNPs and˜15% ligands dispersed in a host matrix.

In an embodiment, a SNP layer may be coated on an optical device such asa LED to improve its emission spectrum. In another embodiment, aseparate SNP layer may be positioned in the optical path of lightemitted by one or more LEDs for the same purpose. In yet anotherembodiment, a layered SNP structure comprising a plurality of differentSNP conversion sub-layers may be coated on a LED. In yet anotherembodiment, the layered SNP structure may be positioned in the opticalpath of light emitted by LEDs. In some embodiments, a SNP layer may bespaced apart from a LED by a coupling layer which may be an air gap, anoptical filter such as short wavelength (UV, blue) pass filter, a longwavelength (e.g. green or red) reflection filter, or an index matchinglayer which minimizes energy loss by reflection. The spacing between theLED and the SNP layer can for example be used to minimize heating due toheat flow from the LED to the SNP layer.

A combination of a LED and a SNP conversion layer or layered structuremay be used in a lighting device (i.e. a domestic light, a signagelight, a vehicle light, a portable light, a back light or any otherlight). In some embodiments, a lighting device may further comprise anoptical element such as a lens, a waveguide, a scatterer, a reflectiveelement, a refractive element or a diffractive element. The opticalelement may be placed between the SNP layer and the light source orbefore the SNP layer in the optical path, or on the sides of the layer(for example a reflective element for using scattered light). In someembodiments, the lighting device may include a combination of two ormore optical elements from the list above in addition to a LED and oneor more SNP layers.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the invention are herein described, by wayof example only, with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of known core/shell particles: (A)core QD/shell QD; (B) RSNP; (C) SNP;

FIG. 2 shows experimental results of optical absorption and emission ofa core/shell QD material vs. a RSNP material used in embodiments of theinvention: (A) for green light; (B) for orange light;

FIG. 3 shows normalized absorption curves of three types of red emittingSNPs having different lengths;

FIG. 4 illustrates schematically the FRET effect in densely-packed QDs,in which it is efficient, and in densely-packed RSNPs, in which it isblocked;

FIG. 5A shows schematically a light conversion device which includes aSNP layer according to one embodiment of the invention;

FIG. 5B shows schematically a light conversion device which includes aSNP layer according to another embodiment of the invention;

FIG. 6A shows schematically a light conversion device which includes aSNP layer according to yet another embodiment of the invention;

FIG. 6B shows schematically a light conversion device which includes aSNP layer according to yet another embodiment of the invention;

FIG. 7 shows schematically a lighting device which includes a SNP layeraccording to an embodiment of the invention;

FIG. 8 shows schematically a lighting device which includes a SNP layeraccording to another embodiment of the invention;

FIG. 9 shows schematically a lighting device which includes a SNP layeraccording to yet another embodiment of the invention.

FIG. 10A shows a LED coupled to a waveguide that has an SNP layerembedded therewithin;

FIG. 10B shows schematically a LED coupled to a waveguide that has anSNP layer positioned on a top surface thereof;

FIG. 11 shows the absorption (dotted line) and the PL (full line) of aconversion layer which comprises 33×7 nm CdSe/CdS RSNPs embedded inpolymer PVB film;

FIG. 12A shows the light spectrum of a lighting device comprising a 455nm blue LED with a conversion layer which comprises 33×7 nm CdSe/CdSRSNPs embedded in a PVB film with added BaSO₄ particles;

FIG. 12B shows the light spectrum of a lighting device comprising a 455nm blue LED with a conversion layer which comprises 27×5.5 nm CdSe/CdSRSNPs embedded in a Silicone RTV film;

FIG. 13A shows the absorption (dotted line) and the PL spectra (fullline) of a dense spin coated red emitting RSNP layer on glass;

FIG. 13B shows the absorption (dotted line) and the PL spectra (fullline) of a dense spin coated green emitting RSNP layer on glass;

FIG. 14A shows the normalized light spectrum of a lighting devicecomprising a broad band LED based element with a SNP conversion layer.

FIG. 14B shows the normalized light spectrum of a lighting devicecomprising a broad band LED based element with another SNP conversionlayer.

FIG. 15 shows a CIE chart with the two outputs of FIGS. 14A, B marked asshown.

FIG. 16 shows the absorption (dotted line) and the PL spectra (fullline) of the SNP film of Example 7.

DETAILED DESCRIPTION

Embodiments of SNP layers, SNP layers used to condition LED light andlighting devices including such layers are now described in more detail.In particular, advantageous properties and features of such layers aredescribed next with reference to FIGS. 2-4. The various SNP layersmentioned below may be prepared using procedures detailed in Examplesbelow.

Reference is now made to FIGS. 2A, B, which show a comparison betweenthe absorption and emission of a known conventional CdSe/ZnS core/shellQD layer and two types of RSNP layers according to embodiments of theinvention: a green emitting RSNP layer (FIG. 2A) and an orange emittingRSNP layer (FIG. 2B). The comparison is between the absorption andnormalized emission of the QD layer vs. the SNP layers having a matchedabsorption at the excitation wavelength of 450 nm. The Green RSNP layerincluded CdSe/CdS core/shell RSNPs with dimensions 4×27 nm(diameter×length), emitting at a center wavelength (CWL) or peakwavelength of 540 nm with a full width half maximum (FWHM) of 29 nm. TheOrange RSNP layer included CdSe/CdS RSNPs with dimensions 5×40 nm, a CWLat 600 nm and FWHM of 28 nm. Both Orange and Green emitting layers wereprepared in a similar way, and both were 190 μm-thick, with diameter of42 mm.

The PL quantum yield (QY) of both QD and RSNP nanoparticles was similarand on the order of 50%. This is a typical value. In other preparedsamples, the QY ranged from 5-100%, more often between 20-90% and evenmore often between 50-80%. The absorption is measured in relativeoptical density (OD) units, where the scale shown is normalized to therange [0 1] for convenience. Significantly, for the green light emittinglayers in FIG. 2A, the OD of the QD layer in the emission wavelengthrange (e.g. 520-550 nm) is 10 times higher than that of the RSNP layer(0.64 vs. 0.065). The OD difference for the orange emitting layers inFIG. 2B is even higher (0.575 vs. 0.037, a factor of ˜15). In otherexamples (not shown), the OD in the emission range of a QD layer wasfound to be 3-30 times higher than that of a RSNP layer. Therefore,losses due to self-absorbance are significant for the QD layer case andnegligible for the RSNP layer case. This property is used in various SNPlayers of the invention (whether densely-packed or not) to provide farsuperior products over existing layers based on quantum dots.

The inventors have further determined that SNP layers of the inventionhave a feature of very efficient “funneling” of energy from blueexcitation to red emission. An SNP layer acts essentially as an “opticalantenna” in the spectral sense. It performs this task much moreefficiently that a regular QD layer, since it has very high absorbancein the blue and strong red PL accompanied by minimal red re-absorption,see FIG. 3.

FIG. 3 shows normalized absorption curves of three types of RSNP layersprepared as described in Example 1 below and comprising in each case redemitting RSNPs (CdSe/CdS) having different overall dimensions and nearlysimilar emission spectra: a curve 300 for 5.8×16 nm RSNPs with 622 nmemission, a curve 302 for 4.5×45 nm RSNPs with 625 nm emission and acurve 304 for 4.5×95 nm RSNPs with 628 nm emission. These curvesillustrate the funneling effect in different conversion layers. Theabsorption curves are normalized to OD 1 at 455 nm. The “absorptionratio” between absorption at 455 nm to that at the emission wavelengthsis respectively 1:5, 1:12 and 1:23 for SNP layers with RSNPs of lengths16, 45 and 95 nm. This shows that the funneling is more efficient forlayers comprising longer RSNPs and that the absorption ratio is“tunable” by varying the RSNP length. Note that for SNPs which are notrod-shaped, a similar tuning can be achieved by increasing the shell tocore diameter ratio. This tunability is very useful in SNP layers sinceit allows the SNP layers to act as the efficient spectral antenna toconvert blue light to red light desired in a light source andapplication. An additional parameter resulting from this specialcharacteristic of SNP layer is that it allows to efficiently balance thelight between the different spectral regions of the visible spectrum(say green-yellow emitted by CE:YAG and the red emitted by SNPs) toobtain light with required characteristic (such as CCT and CRI).

The inventors have further determined that layers with densely-packedSNPs have significantly smaller FRET losses than layers havingdensely-packed QDs. FIG. 4 illustrates schematically the FRET effect indensely-packed conversion layers of QDs and RSNPs. FIG. 4A, for the QDconversion layer case, shows some QDs acting as donors (D, 410) and someacting as acceptors (A, 420), with a typical distance between donor andacceptor denoted by the arrow 430. In such a typical QD conversionlayer, the smaller QDs act as donors to larger QDs which act asacceptors. The typical center-to-center distance is on the order of theFRET distance of ˜10 nm, hence FRET is efficient in such adensely-packed QD conversion layer. FIG. 4B, for the SNP layer case,shows that the special geometry induces on average large distancesbetween a RSNP 440 emitting a colour slightly bluer as compared toanother RSNP 450. The typical core-to-core distance in this case(indicated by 460) is around half of the RSNP length, and is engineeredto exceed the FRET distance, leading to a significantly reducedprobability for FRET processes.

To reiterate, known QDs do not provide such a large distance andconsequently, in a densely-packed layer arrangement, their FRET lossesare inherent, leading to reduced conversion efficiency. In addition, ina densely-packed QD conversion layer, the FRET process leads to a redshift of the emission. In contrast, in a SNP layer as disclosed herein,the emission is maintained at the tailored and desired wavelength,providing the required colour and higher energy efficiency.

To reemphasize, the optical properties of the SNP layers of theinvention provide significant advantages over existing QD conversionlayers due to low re-absorption and small energy transfer losses andlight colour changes. The capability to minimize re-absorption impliesthat higher absorption (by longer optical paths and/or higherconcentration of the SNP) may be used. As a result, significantabsorption of the blue or UV LED light may be achieved and higherefficiency devices are enabled, exemplified by the spectral antennacharacteristic of the SNP conversion layers disclosed herein.

In known QD conversion layers, the formation of clusters of QD materialmay lead to energy losses via FRET, as described above. Clustering canoccur even at low loading, while high loading can occur without densepacking (the latter correlated with extremely high loading). Since QDsare densely-packed in a cluster, the distance between neighbouring QDsis small and energy transfer processes may become significant. Thesewill reduce the emission output and the efficiency of the devices andwill also affect the light colour output. SNP clustering in a conversionlayer does not lead to energy transfer losses, and therefore losses inefficiency or changes in the light colour output are avoided. Therefore,devices based on SNP will function even if clusters are formed. Thisenables the use of thinner layers.

Known QD materials for light conversion are embedded in a host material(matrix) in a low-loading ratio to avoid losses by mechanisms such as byFRET. As a result, a QD conversion layer must be thick (typicallythicker than 100 μm in most cases), yet still contain sufficient amountof material to achieve effective absorption of blue light forconversion, thereby inherently leading to re-absorption losses. Inaddition, for thick layers, manufacturing methods become less accurateand more resource consuming. In sharp contrast, high-loading ratio SNPlayers may be made very thin. For example, thin SNP layers may beproduced using spin-coating deposition techniques, see Examples 4 and 5in which the layers are respectively 510 nm and 230 nm-thick. Ingeneral, for SNP/RSNP conversion layers of the invention, absorption andemission can be controlled to provide tailored colour and opticalcharacteristics, power and efficiency. Densely-packed, high-loading thinSNP layers have an additional advantage in that they may be made withexcellent uniformity over large length scales, from a few millimeters tocentimeters and even more.

High-loading ratio SNP layers may be prepared using a polymer, epoxy orresin matrix, or simply by having a layer of close-packed SNPs. Thepolymer or additive may serve additional purposes such as forencapsulating the optically active nanoparticles to prevent oxidation orphoto-degradation, serve as a medium easy for mechanical integration inthe lighting device and as a medium which can also enhance the lightextraction from the layer due to its refractive index and surfaceroughness. A host material (matrix) can also serve as a matrix fordiffusive particles such as SiO₂, Al, BaSO₄ or TiO₂, which can enhancethe scattering within the layer. The loading ratio may be used tocontrol the refractive index of the SNP layer. Layers of low-loadingratio may have a refractive index as low as 1.5 and even lower, whilelayers with a high-loading ratio may have a refractive index of 1.8 andeven higher. Typically, for polymers with a refractive index of 1.3-1.5,the refractive index will not change up to ˜15% loading ratio.Typically, with ligands, the refractive index may be 1.8 and more.

Tables 1-3 summarize various exemplary embodiments of SNP/RSNPconversion layers made according to the invention. Other embodiments ofconversion layers having advantageous physical parameters and opticalperformance are possible and can be made according to the teachingsdisclosed herein. Therefore, these exemplary embodiments should not beconsidered as limiting the invention in any way.

TABLE 1 Parameters for Red-emitting RSNP conversion layers PL SNP LayerRed Conversion Embedding length Emission Thickness Shift^(d) layer/RSNPtype material [nm] [nm] [μm] AR^(a) _(red) AR^(b) _(green) OD^(c) [nm]CdSe\CdS Ligands^(e) 8-150 580-680 0.1-2 For RSNP For RSNP 0.07- <5ZnSe\CdS length length 2.0 CdSe\CdS\ZnS 8-100 nm, 8-110 nm, CdSe\CdZnSAR > 3.5:1 AR > 2.5:1 CdSe\CdZnS\ZnS For RSNP For RSNP length length 60-60- 150 nm, 100 nm, AR > 7:1 AR > 6:1 CdSe\CdS Polymer^(f) 8-150 580-680   1-5000 For SNP For RSNP 0.07- <5 ZnSe\CdS or length length 2.0CdSe\CdS\ZnS Silicone^(g) 8-100 nm, 8-110 nm, CdSe\CdZnS AR > 3.5:1 AR >2.5:1 CdSe\CdZnS\ZnS For RSNP For RSNP length length 60-150 60-100 nm,nm. AR > 7:1 AR > 6:1 Markings in Table 1: ^(a)AR_(red) is the ratiobetween the absorbance at 455 nm to the maximal absorbance in awavelength range between 580-700 nm, i.e. AR_(red) =(Absorbance_(455 nm)/max(Absorbance_(580-700 nm)); ^(b)AR_(green) is theratio between the absorbance at 455 nm to the maximal absorbance in awavelength range between 520-580 nm, i.e. AR_(green) =(Absorbance_(455 nm)/max(Absorbance_(520-580 nm)); ^(c)OD is measured at455 nm; ^(d)PL Red shift is the difference in nanometers between the CWLmeasured in Toluene at low OD (<0.1) and the CWL measured for thesample; ^(e)Ligands can be selected from list given in definitions;^(f)The polymer can be selected from list given in definitions;^(g)Silicone with suitable optical and mechanical properties can beselected from various commercial suppliers.

TABLE 2 Parameters for Green-emitting RSNP conversion layers PL SNPLayer Red Conversion Embedding length Emission Thickness Shift^(c)layer/RSNP type material [nm] [nm] [μm] AR^(a) _(green) OD^(b) [nm]CdSe\CdS Ligands^(d) 8-150 520-580 0.1-5 For RSNP length 0.07- <5ZnSe\CdS 8-100 nm, 2.0 CdSe\CdS\ZnS AR > 3.5:1 CdSe\CdZnS For RSNPlength CdSe\CdZnS\ZnS 45:150 AR > 7:1 CdSe\CdS Polymer^(e) 8-150 520-580  1-10 For RSNP length 0.05- <5 ZnSe\CdS or 8-100 nm, 2.0 CdSe\CdS\ZnSSilicone^(f) AR > 3.5:1 CdSe\CdZnS For RSNP length CdSe\CdZnS\ZnS45:150, AR > 7:1 Markings in Table 2: ^(a)AR_(green) is the ratiobetween the absorbance at 455 nm to the maximal absorbance in awavelength range between 520-580 nm, i.e. AR_(green) =(Absorbance_(405 nm)/max(Absorbance_(520-580 nm)); ^(b)OD is measured at405 nm; ^(c)PL Red shift is the difference in nanometers between the CWLmeasured in Toluene at low OD (<0.1) and the CWL measured for thesample; ^(d)Ligands can be selected from list given in definitions;^(e)The polymer can be selected from list given in definitions;^(f)Silicone with suitable optical and mechanical properties can beselected from various commercial suppliers.

TABLE 3 Parameters for Green-and Red-emitting SNP conversion layersLayer PL Red Conversion layer/ Embedding Emission Thickness Shift^(d)SNP type material [nm] [μm] AR^(a) _(green) AR^(b) _(red) OD^(c) [nm]CdSe\CdS Ligands^(e) 580- 0.1-5 AR > 3:1 0.07- <5 ZnSe\CdS 680 2.0CdSe\CdS\ZnS CdSe\CdZnS CdSe\CdZnS\ZnS CdSe\CdS Polymer^(f) 580-   1-10AR > 3:1 0.05- <5 ZnSe\CdS or 680 2.0 CdSe\CdS\ZnS Silicone^(g)CdSe\CdZnS CdSe\CdZnS\ZnS CdSe\CdS Ligands^(e) 520- 0.1-5 AR > 3:1 0.07-<5 ZnSe\CdS 580 2.0 CdSe\CdS\ZnS CdSe\CdZnS CdSe\CdZnS\ZnS CdSe\CdSPolymer^(f) 520-   1-10 AR > 3:1 0.05- <5 ZnSe\CdS or 580 2.0CdSe\CdS\ZnS Silicone^(g) CdSe\CdZnS CdSe\CdZnS\ZnS Markings in Table 3:^(a)AR_(green) is the ratio between the absorbance at 405 nm to themaximal absorbance in a wavelength range between 520-580 nm, i.e.AR_(green) = (Absorbance_(405 nm)/max(Absorbance_(520-580 nm)),^(b)AR_(red) is the ratio between the absorbance at 455 nm to themaximal absorbance in a wavelength range between 580-680 nm, i.e.AR_(red) = (Absorbance_(455 nm)/max(Absorbance_(580-680 nm)); ^(c)OD ismeasured at 455 nm for nanoparticles emitting at 580-680 nm and at 405nm for nanoparticles emitting at 520-580 nm; ^(d)PL Red shift is thedifference in nanometers between the CWL measured in Toluene at low OD(<0.1) and the CWL measured for the sample; ^(e)Ligands can be selectedfrom list given in definitions; ^(f)The polymer can be selected fromlist given in definitions; ^(g)Silicone with suitable optical andmechanical properties can be selected from various commercial suppliers.

FIG. 5A shows schematically a lighting device 500 a which includes a SNPlayer 502 a according to one embodiment of the invention. Light producedby a suitable source 504 a (exemplarily a LED emitting UV light) isdirected at SNP layer 502 a. Layer 502 a comprises SNPs that convert thelight from blue and/or UV to longer wavelengths. Different populations(types) of SNPs (having different cores or shell sizes or materials)will emit different colours. The colours emitted by the SNP layer may becombined with the light produced by source 504 a or used independentlyto form different light combinations. In order to improve and tune thespectral properties of the emission, more than one type of SNPs can beused, e.g. mixtures of blue, green and red emitting SNPs (their lightexemplarily marked RGB). The various colours may be chosen so as toprovide white light. Other colour combinations, as desired for aspecific lighting application, can be generated by tailoring the SNPconversion layer.

FIG. 5B shows schematically a lighting device 500 b which includes a SNPlayer 502 a according to another embodiment of the invention. In thisembodiment, light produced by a suitable source 504 b (exemplarily a LEDemitting blue light) remains partially un-converted (i.e. passes throughunaffected) by a SNP layer 502 b. Layer 502 b incorporates SNPs thatconvert the light from blue and shorter wavelengths to green and red.Layer 502 b further incorporates diffusive structures or particles thatspread and mix the unabsorbed light in a tailored pattern, to conformwith the spatial and optical characteristics of the photoluminescence ofthe SNPs incorporated therein. That is, these structures scatter boththe incoming blue light and the SNP-emitted light such that the combinedlight has the same angular diversion when it exits the SNP layer as a“white” light. In addition, the white has high quality green and redlight added to the LED blue light to provide a large colour gamut for adisplay backlight.

FIG. 6A shows schematically a lighting device 600 a which includes a SNPlayer 602 a according to yet another embodiment of the invention. Inthis embodiment, a “colour mixture light” source 604 a is improved orcorrected by SNP layer 602 a. Layer 602 a includes a plurality of SNPspecies which may have different cores or shell sizes, differentmaterials and/or different spectral properties. The SNPs act to convertthe colour mixture light source into an improved colour mixture light.In an embodiment, the improved colour mixture light output from thelighting device can be “white light” with a CCT in the range of2500-6000K with high CRI. In another embodiment, the improved colourmixture light can be white light with a CCT in the range of 2700-4500Krange with high CRI. The source light may be white light with high CCT(for example 5000-10000K). Alternatively, it may be a light combinationwhich cannot be defined as white light but which includes light in therange of the visible spectrum. The improvement includes for exampleaddition of red light to the emission, thereby providing a lower CCT andbetter CRI.

FIG. 6B shows schematically a light conversion device 600 b whichincludes a SNP layer 602 b according to yet another embodiment of theinvention. In this embodiment, layer 602 b includes, in addition to aplurality of SNP species such as in layer 602 a, diffusive structures orparticles that spread and mix the un-absorbed light from source 604 b ina tailored pattern to produce a further improved colour mixture light.

In alternative embodiments, the lighting device can include several SNPlayers, each providing a separate function, may be used instead of asingle SNP layer. Scattering and controlling the transmissioncharacteristics (e.g. homogenization) of transmitted and emitted lightmay be achieved by incorporating in one or more of the SNP conversionlayers either refractive particles such as small SiO₂ beads orreflective particles such as metal particles or light diffusingparticles such as BaSO₄ and TiO₂ by adding a patterning (e.g. diffusive)layer, or by patterning the surface of at least one of the layers.

FIG. 7 shows schematically a lighting device 700 which includes a SNPconversion layer according to an embodiment of the invention. Device 700includes a blue or UV LED light source, an optional spacer layer (or airas spacer) 704, a SNP conversion layer 706, an optional encapsulationlayer 708, an optional transmissive optical element 710 for lightextraction to desired directionality, an optional refractive elementsuch as a lens 712 to collimate or focus the light, and an optionalreflective element 714 placed behind and around the LED element tocollect and direct emission from large angles to the correct outputdirection. In some embodiments, the high refractive index of a SNP layerwith a high-loading ratio is preferred in order to increase the lightextraction from the LED chip.

FIG. 8 shows schematically a lighting device 800 which includes a SNPlayer according to another embodiment of the invention. In device 800,an optical filter 806 is between a SNP layer 802 and a LED emitting chip804. Optical filter 806 is a filter which transmits short wavelength 820(e.g. blue or UV) light and reflects longer wavelength (e.g. green orred) light 822, thereby enabling light recycling and a more efficientdevice. While the light recycling increases the optical path of theemitted light in the SNP layer, due to the low self-absorbance, anyextra loss would be minimized. In contrast, with a QD layer, the extraloss will be significant. Optical elements between the light source andthe SNP layer may also be used to shape or otherwise control the lightsource characteristics. Like device 700, device 800 further comprises anoptional transmissive optical element 810 for light extraction todesired directionality, an optional refractive element such as a lens812 to collimate or focus the light, and an optional reflective element814 placed behind and around the LED element to collect and directemission from large angles to the correct output direction. Placing theSNP layer at a distance from the LED element can diminish the lightintensity at the SNP layer and the temperature level thereby increasingits durability.

FIG. 9 shows schematically a lighting device 900 which includes a SNPlayer according to yet another embodiment of the invention. Device 900includes a SNP layer shaped to fit in a curved optical element 902,serving to colour convert and also to diffuse the light, a LED 904 andadditional layers 906 used for example for spatial patterning or opticalfiltering (e.g. additional UV filtering). SNP layers can be thin yetefficient, which represents a significant advantage over theperformances of thick QD conversion layers.

FIG. 10A shows a LED 1002 a coupled to a waveguide assembly 1004 a thathas an SNP layer 1006 a embedded therewithin. The waveguide includes areflecting layer at a bottom 1008 a (which may be diffusive orreflective, patterned or homogenous) and an optional light extractionlayer 1010 a. FIG. 10B shows schematically a LED 1002 b coupled to awaveguide assembly 1004 b that has an SNP layer 1006 b positioned on atop surface 1008 thereof and an optional light extraction layer 1010 b.In both embodiments, the SNP layer is shown excited by light coming fromemitted by the LED through an edge 1012 a or 1012 b of the waveguide. Aslight propagates in the waveguide, it passes through the SNP layer againand again. Light converted in the SNP layer is then transmitted acrossthe waveguide over a relatively great distance, which can be in themillimeters to centimeters to tens of centimeters range. In thisapplication, the low self-absorbance of the SNP layer may be critical,since the light travels over a long optical path. The reflective and/ordiffusive optical elements (1008 a, 1008 b 1010 a, 1010 b and 1012 b)may be placed at all areas of the device where the light can be emittednot in the needed direction. These elements will return the light intothe waveguide and increase its efficiency

EXAMPLES Example 1 Lighting Device with RSNP Conversion Layer withinPolymer Host Providing Red Light

RSNPs were synthesized following similar procedures to those describedin L. Carbone et al. “Synthesis and Micrometer-Scale Assembly ofColloidal CdSe/CdS Nanorods Prepared by a Seeded Growth Approach” NanoLetters, 2007, 7 (10), pp 2942-2950. In a first step, CdSe cores withdiameter of 3.8 nm were synthesized. In a second step, red emittingCdSe/CdS RSNPs were synthesized using the CdSe cores as seeds. Theresulting RSNPs had dimensions of 33×7 nm with an emission maximum is at635 nm with FWHM of 30 nm when measured in a Toluene solution.

A RSNP conversion layer was prepared as follows: 0.5 g of Poly(vinylbutyral-co-vinyl alcohol-co-vinyl acetate) (PVB), a resin usually usedfor applications that require strong binding, optical clarity, adhesionto many surfaces, toughness and flexibility and commercially availablefrom Sigma-Aldrich (3 Plaut St., Rabin Park, Rehovot 76100, Israel) weredissolved in 4 ml Toluene. 12 mg of RSNPs were dissolved in 1 ml Tolueneto form a RSNP solution. The RSNP solution was added to the polymermixture while stirring. The mixture was transferred to a pattern vesselwhich was inserted into a dessicator and vacuumed for 15 hours, afterwhich the mixture became solid. The resulting film thickness was 190 μm.The optical characteristics of the conversion layer are presented inFIG. 11, which shows the absorption (dotted line) and the PL (full line)spectra. The emission maximum is at 635 nm, with a FWHM of 30 nm. Theabsorption OD is 1.18 at 455 nm, 0.07 at 540 nm and <0.046 at 600-750nm, i.e. 25 times smaller than the OD at 455 nm. This RSNP layertherefore funnels light from blue to red emission.

The RSNP layer was incorporated in a lighting device similar to that ofFIG. 5A. In the lighting device, a UV LED at 360 nm was used toilluminate the RSNP layer, providing light output in the red, at 635 nm.Negligible UV output was detected, as the UV light was absorbed andconverted very effectively by the RSNP layer.

Example 2 Lighting Device with Diffusive RSNP Conversion Layer withinPolymer Host Providing Combination of Blue and Red Light

A diffusive RSNP layer was prepared using the procedure in Example 1,with a modification that 1.3 mg of RSNP was dissolved in 1 ml Tolueneand that after the 10 minutes stirring of RSNPs in polymer, 5 mg ofBaSO₄ particles were added to the solution and stirred for another 15minutes. The resulting film had diffusive properties that enhanced theoptical emission and increased the extraction of the light in a requireddirection.

The RSNP layer was incorporated in a lighting device as shown in FIG.5B. A blue LED emitting at 455 nm was used to illuminate the RSNP layer.The lighting output was measured and the light spectrum is presented inFIG. 12A, which shows a combination of a blue remnant from the blue LEDand a red component from the RSNP conversion layer.

Example 3 Lighting Device with RSNP Conversion Layer within Silicone RTVProviding Combination of Blue and Red Light

A RSNP layer in Silicone RTV was prepared as follows: 1 g of RTV615A(Momentive, 22 Corporate Woods Boulevard, Albany, N.Y. 12211 USA) wasstirred with 0.1 g of RTV615B for 10 min. 1.5 mg of CdSe/CdS RSNPs withoverall dimensions of 27×5.5 nm emitting at 635 nm was dissolved in 250μl Toluene. The RSNP solution was added to the silicone mixture whilestirring, then vacuumed until no bubbles remained. The solution was thendeposited on a glass substrate and sandwiched using another glasssubstrate. 600 μm-thick spacers were positioned between the two glasssubstrates to obtain the desired film thickness. The sandwichedstructure was then placed on a hot plate at 150° C. for 15 minutes,after which the solution became solid. The measured film thickness was600 μm.

The RSNP layer was incorporated in a lighting device as shown in FIG.5B. A blue LED emitting at 455 nm was used to illuminate the RSNP layer.The lighting output was measured and the light spectrum is presented inFIG. 12B, which shows a combination of a blue remnant from the blue LEDat 455 nm and a red component from the RSNP layer at 635 nm with a FWHMof 30 nm.

Example 4 Lighting Device with Thin Dense Spin-Coated RSNP ConversionLayer Providing Red Light

A dense RSNP layer was prepared as follows: first prepared was asolution of 35×5.4 nm CdSe/CdS RSNPs, emitting at 635 nm, in Toluenewith 1:4 weight/volume (mg/μL) ratio. 20 μL of the solution was dropcast on a soda lime glass substrate and spread by spin coating at 2000rpm. The deposited film was measured to have an absorbance OD of 0.51 at455 nm, and OD of 0.9 at 360 nm. The thickness was 0.510 um as measuredby a profilometer. FIG. 13A shows the absorption (dotted line) and thePL (full line) spectra of this RSNP layer. The emission maximum is at633 nm, with a FWHM of 33 nm. The absorption OD is 0.96 at 360 nm, 0.5at 455 nm, 0.035 at 540 nm and 0.025 at 600-750 nm, the latter 20 timessmaller than the OD at 455 nm.

The RSNP layer was incorporated in a lighting device as shown in FIG. 5AA UV LED at 360 nm was used to illuminate the RSNP layer, and providedlight output in the red at 633 nm (not shown).

Example 5 Lighting Device with Thin Dense Spin-Coated RSNP ConversionLayer Providing Green Light

A dense RSNP layer was prepared as follows: A solution of green emitting20×3.5 nm CdSe/CdS RSNPs in Toluene with 1:5 weight/volume (mg/μL) ratiowas prepared. 20 μL of solution containing the RSNPs was drop cast on asoda lime glass substrate and spread by spin coating at 2000 rpm. Thedeposited film was measured to have an absorbance OD of 0.07 at 455 nmand a thickness of 230 nm as measured by a profilometer. FIG. 13B showsthe absorption (dotted line) and the PL (full line) spectra of this RSNPlayer. The emission maximum is at 540, with a FWHM of 33 nm. Theabsorption OD is 0.165 at 360 nm, and 0.008 at 540 nm, the latter 20times smaller than the OD at 360 nm.

The RSNP layer was incorporated in a lighting device as shown in FIG.5A. A UV LED at 360 nm was used to illuminate this RSNP layer, andprovided light output in the red, at 540 nm (not shown).

Example 6 Lighting Devices with RSNP Conversion Layers Providing WhiteLight

Two RSNP layer samples were prepared using the methods described abovefor PVB with scatterers (example 2) RSNP layer CL14A had 10 mg of redemitting RSNPs and 25 mg of BaSO₄, inserted into 0.5 g of PVB. RSNPlayer CL14B had 20 mg of red emitting RSNP and 25 mg of BaSO₄, insertedinto 0.5 g of PVB. Each of the two samples was 190 μm-thick and had adiameter of 42 mm.

The RSNP layers were incorporated in two lighting devices as shown inFIG. 6B. In both lighting devices, the RSNP layers were placed at theaperture of a LED module composed of a blue LED source and a speciallyprepared Ce:YAG based phosphor layer in a silicone matrix. The lightoutput was measured and is shown in FIGS. 14A and B, for a lightingdevice with layers CL14A and CL14B, respectively. The light seen iscomposed of contributions of blue light from the 455 nm LED, a broadpeak around 580 nm for the Ce-YAG based phosphor and red light from theRSNP layer. The CIE 1931 coordinates were calculated and the location ofthe two lighting devices on the CIE Chromaticity Diagram is shown inFIG. 15. The CCT for CL14A is 3420 K and for CL14B is 2730 K while theCRI for CL14A is 95 and for CL14B is 92.

Example 7 Lighting Devices with SNP Conversion Layers Providing WhiteLight

The SNP used was a non-rod shaped CdSe\CdZnS SNP with CdSe core diameterof 3.9 nm and overall diameter of 8.9 nm. 0.5 gr of PVB were dissolvedin 4 ml Toluene. 2 mgr of SNP was dissolved in 1 ml Toluene. The SNPsolution was added to the polymer mixture while stirring. After 10 minstirring, the mixture had a shining glow. The mixture was thentransferred to a pattern vessel which was inserted in a dessiccator andvacuumed for 15 hours, after which the mixture became solid. The finalfilm thickness was 190 um. FIG. 16 shows the absorption (dotted line)and the PL spectra of this film. The CWL is at 626 nm and the FWHM is 33nm. The absorption ratio between absorption at 455 to maximum absorptionin 600-700 nm range is 1:6 (0.156 to 0.026). The SNP layer wasincorporated in a lighting device as shown in FIG. 5A. A UV LED at 360nm was used to illuminate this RSNP layer, and provided light output inthe red, at 626 nm (not shown).

In conclusion, various embodiments of the invention provide devicesincorporating novel conversion layers based on SNPs. Conversion layersdisclosed herein are characterized by low re-absorption in the emissionregion compared with the absorbance in the exciting wavelength. SNP/RSNPconversion layers disclosed herein are suitable for enhancing theproperties of LED devices to provide white emission with a CCT<4000Kwith a high CRI>80 and even >85, in particular a CCT<3500 and even aCCT-2700K with CRI>89. Polymer embedded SNP conversion layers of theinvention can be further prepared to provide a white colour for displayapplications composed of three or more primary colours with a narrowFWHM<60 nm and even a FWHM<40 nm.

The invention has been described with reference to embodiments thereofthat are provided by way of example and are not intended to limit itsscope. The described embodiments comprise different features, not all ofwhich are required in all embodiments of the invention. Some embodimentsof the invention utilize only some of the features or possiblecombinations of the features. Variations of embodiments of the describedinvention and embodiments of the invention comprising differentcombinations of features than those noted in the described embodimentswill occur to persons of ordinary skill in the art. The scope of theinvention is limited only by the following claims.

All patents, patent applications and publications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individual patent,patent application or publication was specifically and individuallyindicated to be incorporated herein by reference. In addition, citationor identification of any reference in this application shall not beconstrued as an admission that such reference is available as prior artto the present invention.

The invention claimed is:
 1. An optical conversion layer for use in alighting device in which a light source emits light of a firstwavelength, the conversion layer comprising at least one type ofsemiconductor nanoparticles having a central emission wavelength (CWL)in the range 520-580 nm, the conversion layer characterized by having anoptical density (OD) between 0.05 and 2 at 405 nm and an absorptionratio (AR) of absorbance at 405 nm to a maximum value of absorbance inthe range 520-600 nm greater than 3.5:1, wherein the conversion layermay be used to convert at least part of the light of the firstwavelength into light of a second wavelength longer than the firstwavelength.
 2. The conversion layer of claim 1, wherein thenanoparticles are seeded nanoparticles (SNPs).
 3. The conversion layerof claim 2, wherein the seeded nanoparticles are nanorod seededparticles (RSNPs).
 4. The conversion layer of claim 2, furthercomprising a host material which incorporates the nanoparticles.
 5. Theconversion layer of claim 4, wherein the host material is a polymer or asilicone.
 6. The conversion layer of claim 5, wherein the layer isthinner than about 5000 μm.
 7. The conversion layer of claim 1, whereinthe AR is higher than 7:1.
 8. The conversion layer of claim 1, furthercharacterized by at least one of the following: (1) having aphotoluminescence (PL) shift smaller than about 5 nm, wherein the PLshift represents the difference between a CWL measured in Toluene atOD<0.1 and a CWL measured in the conversion layer; (2) comprising atleast one excess organic ligand not bound to any SNP surface.
 9. Theconversion layer of claim 1, wherein the layer is thinner than about 5μm.
 10. A lighting device comprising: a) a light source which emitssource light of a first wavelength; and b) a conversion layer configuredaccording to claim 1, the conversion layer being used to convert atleast part of light of the first wavelength into light of a secondwavelength longer than the first wavelength.
 11. The lighting device ofclaim 10, wherein the nanoparticles are seeded nanoparticles.
 12. Thelighting device of claim 11, wherein the seeded nanoparticles arenanorod seeded particles.
 13. The lighting device of claim 10, whereinthe light source is at least one light emitting diode (LED) and whereinthe conversion layer enhances the lighting device light output toprovide one of the following: (1) white light with a CCT<10000K and witha CRI>70; and (2) white light with a CCT<4500K and with a CRI>80.
 14. Anoptical conversion layer for use in a lighting device in which a lightsource emits light of a first wavelength, the conversion layercomprising at least one type of semiconductor nanoparticles having acentral emission wavelength in the range 520-700 nm, the conversionlayer characterized by having an optical density between 0.07 and 2.5 at405 nm and an absorption ratio of absorbance at 405 nm to a maximumvalue of absorbance in the range 520-700 nm greater than 3:1, whereinthe conversion layer may be used to convert at least part of the lightof the first wavelength into light of other wavelengths longer than thefirst wavelength.
 15. A lighting device comprising the opticalconversion layer of claim
 14. 16. The lighting device of claim 14,wherein the conversion layer is adapted to pass through an unconvertedpart of the source light of the first wavelength.
 17. A lighting devicecomprising: a) a light source which emits source light of a firstwavelength; and b) the conversion layer configured according to claim14, the conversion layer being used to convert at least part of light ofthe first wavelength into light into other wavelengths longer than thefirst wavelength.
 18. The lighting device of claim 17, wherein thenanoparticles are seeded nanoparticles.
 19. The lighting device of claim18, wherein the seeded nanoparticles are nanorod seeded particles. 20.The lighting device of claim 17, wherein the light source is at leastone light emitting diode and wherein the conversion layer enhances thelighting device light output to provide one of the following: (1) whitelight with a CCT<10000K and with a CRI>70; (2) white light with aCCT<4500K and with a CRI>80.
 21. The conversion layer of claim 14,wherein the seeded nanoparticles are rod shaped nanoparticles (RSNPs).22. The conversion layer of claim 14, further characterized by at leastone of the following: (i) having an AR between absorbance at 455 nm anda maximum value of absorbance in the wavelength range of 520-580 nmgreater than 7:1; (ii) having a photoluminescence (PL) shift smallerthan about 5 nm, wherein the PL shift represents the difference betweena CWL measured in Toluene at OD<0.1 and a CWL measured in the conversionlayer; and (iii) comprising at least one excess organic ligand not boundto any SNP surface.
 23. The conversion layer of claim 14, wherein thelayer is thinner than about 2 μm.
 24. The conversion layer of claim 14,wherein the semiconductors are selected from the group consisting ofII-VI, III-V IV-VI and III-VI₂ semiconductors.
 25. The conversion layerof claim 14, wherein the nanoparticles have at least one of thefollowing configurations: (a) comprise a core/shell structure withmaterials selected from the group consisting of CdSe/CdS, CdSeS/CdS,ZnSe/CdS, ZnCdSe/CdS, CdSe/CdZnS, CdTe/CdS, InP/ZnSe, InP/CdS, InP/ZnSand CuInS₂/ZnS; (b) a core/double shell structure with materialsselected from the group consisting of CdSe/CdS/ZnS, CdSe/CdZnS/ZnS,ZnSe/CdS/ZnS, InP/ZnSe/ZnS, InP/CdS/ZnS and InP/CdZnS/ZnS.
 26. Theconversion layer of claim 14, further comprising a host material whichincorporates the nanoparticles.
 27. The conversion layer of claim 26,wherein the host material is a polymer or a silicone.
 28. The conversionlayer of claim 27, wherein the layer is thinner than about 5000 μm. 29.The conversion layer of claim 14, comprising at least two types ofnanoparticles, a first type comprising nanoparticles having centralemission wavelength in the range of 520-600 nm and a second type ofnanoparticles having central emission wavelength in the range of 580-700nm.
 30. The conversion layer of claim 14, further comprising at leastone excess organic ligand not bound to any SNP surface.
 31. Theconversion layer of claim 14, being thinner than about 5000 μm.
 32. Theconversion layer of claim 14, being thinner than about 50 μm.
 33. Theconversion layer of claim 14, wherein the at least one type ofsemiconductor nanoparticles includes a plurality of types, each typeperforming conversion into light of a different wavelength longer thanthe first wavelength.
 34. The conversion layer of claim 14, furthercharacterized by at least one of the following: (i) having an AR betweenabsorbance at 455 nm and a maximum value of absorbance in the wavelengthrange of 580-700 nm greater than 6:1; (ii) having a photoluminescence(PL) shift smaller than about 5 nm, wherein the PL shift represents thedifference between a CWL measured in Toluene at OD<0.1 and a CWLmeasured in the conversion layer; and (iii) comprising at least oneexcess organic ligand not bound to any SNP surface.
 35. The conversionlayer of claim 14, further characterized by an absorption ratio ofabsorbance at 405 nm to a maximum value of absorbance in the range520-700 nm greater than 6:1.
 36. An optical conversion layer for use ina lighting device in which a light source emits light of a firstwavelength, the conversion layer comprising at least one type ofsemiconductor nanoparticles having a central emission wavelength in therange 520-700 nm, the conversion layer characterized by having anoptical density between 0.05 and 2.5 at 405 nm and an absorption ratioof absorbance at 405 nm to a maximum value of absorbance in the range520-700 nm greater than 3:1, wherein the conversion layer may be used toconvert at least part of the light of the first wavelength into light ofother wavelengths longer than the first wavelength.